Make the dough:

1. Whisk together the flour, sugar, and salt in a large bowl. Add the cold butter cubes and work them in with your fingertips or a pastry cutter until the mixture resembles coarse crumbs with some pea-size butter pieces remaining.
2. Drizzle in 3 tbsp ice water and stir with a fork until the dough just comes together. Add the remaining tablespoon of water only if needed. Divide into four equal balls, flatten each into a disk, and wrap in plastic. Refrigerate at least 1 hour (or overnight).

Make the filling:

1. In a large bowl, toss together the strawberries, sugar, cornstarch, salt, lemon zest, and vanilla. Let the berries macerate while you roll out the dough. They will release juice as they sit — when filling the galettes, leave any pooled liquid behind in the bowl.

Assemble the galettes:

1. Grease four mini pie pans with butter (or line a baking sheet with parchment paper). On a lightly floured surface, roll each dough disk into an 8-inch round. Transfer into the pie pans, pressing lightly to fit (or lay flat on the parchment).
2. Spoon the strawberry mixture into the center of each round, leaving about 1 inch of dough around the edge and leaving excess liquid in the bowl. Gently fold the crust edge up and over the edge of the filling, pleating as you go. It should look rustic — don’t try too hard.

Bake:

1. Whisk the egg white and water together and brush liberally over the folded crust. Sprinkle the crust with sugar.
2. Bake at 400°F (205°C) for 30–35 minutes, checking at 27 minutes. The crust should be golden brown and the filling bubbling. The dough firms further as it cools — err on the early side.
3. Serve warm with ice cream or whipped cream.